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Chemical Kinetics

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EKO UNIVERSITY OF MEDICINE AND HEALTH SCIENCES, IJANIKIN, LAGOS

FACULTY OF BASIC MEDICAL SCIENCES


DEPARTMENT OF CHEMISTRY

COURSE TITLE: GENERAL CHEMISTRY II


COURSE CODE: CHM 102
(LECTURE NOTE AND ASSIGNMENT)

BY

ERESANYA .O.I
CHEMICAL KINETICS (PART 1)

LECTURE 5
CHEMICAL KINETICS
Chemical kinetics studies the rate at which a chemical process occurs. Apart from information
about the speed at which reactions occur (reaction rate), it also sheds light on the reaction
mechanism (details of all the steps involved in the reaction).

REACTION RATES
Reaction rate can be defined as a change in the concentration of a reactant or a product per unit
time. The unit is moldm-3s-1.
It is a measure of speed of a chemical reaction. There are two extreme types of reactions in
terms of rate:
(a) Slow reactions; These take a long time (hours, days, weeks, months, etc) to complete e.g.
weathering of rocks (breaking down of rocks), formation of coal, rusting of iron and so on.
(b) Instantaneous (fast) reactions; These take a very short time (microseconds or seconds) to
complete. e.g. neutralization reaction, combustion of LPG gas, etc.

Series of experiments have indicated that the rate of a reaction changes with time. The beginning
of a reaction usually has the greatest rate and this rate decreases progressively till the end of the
reaction. It means that a rate must be identified with a specific time.
The measurement of the reaction rate is based on the disappearance of a reactant or
appearance of a product with increasing reaction time.
The rusting of barbed wire indicated below occurs after many years (slow reaction):
Instantaneous reaction
Factors affecting Reaction Rates
The following factors control reaction rates:
(a) Temperature (b) Concentration/ Pressure (c) Presence of Light (d) Catalyst
(e) Nature of Reactants.
(a) Temperature: An increase in temperature typically increases the rate of reaction.
An increase in temperature will raise the average kinetic energy of the reactant
molecules.
Therefore, a greater proportion of molecules will have the minimum energy necessary
for an effective collision.
The figure below indicates Temperature and Reaction Rate.
The relationship between reaction rate and temperature is expressed in the Arrhenius
equation:
Rate = A exp ()

= Ae(- )

Where A = pre-exponential factor or collision factor (a constant)


Ea = activation energy (kJmol-1)
R = gas constant (8.314 Jmol-1K-1)
T = absolute temperature
The Arrhenius equation is useful for the determination of the activation energy of a
reaction. This can be done by a graphical method or by studying the reaction at two
different temperatures, using the expression:

Log10 [] x }

Where Ea = activation energy (kJmol-1)


R = gas constant (8.314 Jmol-1K-1)
K2 and K1 = rate constants
(b) Concentration / Pressure: The reaction rate is directly proportional to the
concentration of the reactants. Hence, when the concentration of the reactants is
increased, the parallel result is fast increase in the concentration of the products.
However, effect of pressure is negligible on the volumes of solids and liquids. Pressure
affect most reactions involving gaseous molecules.
When the pressure of a reacting gaseous molecules are increased, the concentration of
the reactants is increased and this in turn has positive effect on the concentration of the
products.
The quantitative relationship between the rate of a reaction and the concentrations of
reactants is expressed in the rate law or rate equation. For a general reaction
A + B Products
It is found experimentally that:
Rate [A]x [B]y
Rate = k [A]x [B]y
The equation is known as general rate law or rate equation.
Where;
K (a proportionality constant) = specific rate constant
x = exponent to which the concentration of A is raised or the order of the reaction
with respect to A.
y = exponent to which the concentration of B is raised or the order of the reaction with
respect to B.
The sum of exponents x and y gives the overall order or total order of the reaction.
For instance;
x + y = 0 Zero-order reaction

x + y = 1 -order reaction

x + y = 2 -order reaction

x + y = 3 third-order reaction, etc


It should be noted that the order of a reaction can not be deduced from the stoichiometry
(balanced equation) of the reaction. It is actually an experimentally determined quantity.

(c) Presence of Light: Light can speed up some chemical reactions. Such reactions are
called photochemical reactions. It can also alter the cause of others.
For instance, the reaction between hydrogen and chlorine is negligible without light, moderate
in day light and explosive in bright light.
The light activates the reacting particles by breaking their intermolecular bonds and thus
creating active centers. Other examples of photochemical reactions are:
(i) the conversion of silver halide to grey metallic silver, used in photography.
2AgBr + Sunlight 2Ag + Br2
(ii) photosynthesis in plants
(iii) the decomposition of hydrogen peroxide
(d) Catalyst: A catalyst is a substance that accelerates a reaction by participating in it
without being consumed. Catalysts provide an alternate reaction pathway which has
lower activation energy to obtain products.
They are critical to many biochemical reactions. Positive catalyst are used to reduce the
reaction time of many hitherto slow reactions while the negative ones (inhibitors) are
used to suppress unwanted reactions.
Most of the catalysts are specific in their actions , hence, they work on certain reactions
or substances. In addition, most of the catalysts operate well when used in little
quantities. They do not affect the yield of a reaction. Example of catalysts include:
(i) transition metals or their compounds e.g. iron, nichel, palladium, platinum,
manganese (iv) oxide (MnO2) and vanadium(V)oxide (V2O5)
(ii) mineral acids e.g. conc H2SO4, H3PO4 and HCl
(iii)alkalis e.g. KOH, NaOH
(iv) enzymes (biological catalysts) e.g. diastase, maltase, zymase, invertase.
(v) micelles (aggregation of surfactant or detergent molecules in aqueous solution).
The figure below indicates a catalyzed reaction
(e) Nature of Reactants: The nature inform of physical state of matter, bond
type or surface area exposed affects the speed of a reaction. For a substance
in different physical states, the order of reactivity is:
Solid phase < Liquid phase < Gas phase
The solid phase is very slow; the gas phase is very fast and the speed of the
liquid phase is moderate. Many reactions , therefore, are made to take place in
liquid systems. For instance, a liquid reactant is interacted with another liquid
reactant or a solid reactant is first dissolved in a solvent before reacting it with
another reactant.
Also, the type of bonds present in reactants affects reaction rates. For instance,
ionic compounds in aqueous solutions react faster than their covalent
counterparts. This is due to the facts that ionic compounds can produce mobile
ions in solution. The mobile ions can easily acquire energy equal to or greater
than the activation energy of the reaction.
Most reactions require a surface on which reactants come into contact. The
larger the surface area the faster the reaction becomes. For the same masses
of a solid substance (in different forms such as lump, granule and powder), the
smaller the particle size of each form, the larger the surface area exposed to
reaction. Therefore, the order of increasing surface area (which is also the
order of speed of reaction) is:
Lumps < Granules < Powder
ASSIGNMENT
Answer True (T) or False (F) to questions 1-10 below
(1) The reaction rate at the beginning of a reaction is usually the smallest.
(2) Both the presence of a catalyst and temperature affect reaction rates by changing the
amount of activation energy needed.
(3) An increase in concentration is related to the number of collisions inversely.
(4) The reaction speed of 78.9g of iron nails with 120cm3 of 2.5 moldm-3 HCl is the same as
that of 78.9g of iron fillings with 120cm-3 of 2.5 moldm-3 HCl.
(5) The temperature at which a reaction is carried out proportionally affects the number of
reacting particles that acquire the activation energy of the reaction.
(6) The presence of a catalyst affects the pathway of a reaction.
(7) The rate law of a reaction is a mathematical expression showing the relationship
between the reaction rate and the concentrations of the reactants.
(8) Possession of adequate energy is the only condition for obtaining effective collisions
between reactant particles.
(9) The overall order of a reaction can be deduced from the balanced equation of the
reaction.
(10) Activation energy is the energy needed by particles of reactants for effective collisions.

11- 30 questions are objective questions


(11) Which of the reactions of marble with dilute HCl is fastest? (a) 5g of marble lump at
500C
(b) 5g of marble lump at 250C (c) 5g of marble powder at 500C (d) 5g of marble powder at
0
(12) The rate law for a certain reaction is R = k[A]1[B]2, the overall order of the reaction is (a) 2
(b) 3 (c) 1 (d) 0 (e) 4
(13) Consider the reaction:
H2(g) + I2(g) 2HI(g)
Which of the following would decrease the number of effective collisions of molecules of the
reactants? (a) Increase in pressure (b) Rise in temperature (c) Increase in the volume of the
reaction vessel (d) Introduction of a catalyst into the system.
(14) Which of the following factors will slow down the speed of the reaction represented by the
equation below?
2H2O2(l) 2H2O(l) + O2(g)
(a) Exposure of reaction vessel to sunlight (b) Addition of distilled water to the reaction vessel
(c) Increase in temperature (d) Presence of manganese (IV) oxide
(15) N2(g) + 3H2(g) 2NH3(g)
In the chemical reaction above, the substance that will increase the rate of production of
ammonia is (a) platinum (b) vanadium (V) oxide (c) finely divided nickel (d) finely
divided iron
(16) The reaction between 12.3g of zinc granules and 1.5 moldm-3 HCl can be made to proceed
faster by (a) using 0.50 moldm-3 acid instead (b) adding 1.0 moldm-3 NaOH (c) cooling the
reaction vessel (d) adding some concentrated HCl.
(17) A set of conditions that can quicken a chemical reaction is (a) low temperature and high
pressure (b) high temperature, presence of a catalyst and small reactant surface area (c)
presence of a catalyst and darkness (d) high temperature, high reactant concentration and
large reactant surface area.
(18) The times taken for the reaction between sodiumthiosulphate and hydrochloric acid to liberate
iodine at various temperatures are given in the table below:

Temperature (0C) 30 40 50
Time (seconds) 80 40 20

It can be deduced from the results that:


(a) for 100C rise in temperature, the reaction rate is doubled (b) for a 10 0C rise in temperature, the
reaction rate is tripled (c) for a 10 0C rise in temperature, the reaction rate is halved (d) for a
100C rise in temperature, the reaction rate is quadrupled.

(19) The rate of a reaction is proportional to the number of effective collisions occurring per second
between the reactants. This statement is associated with the (a) kinetic theory (b) rate law (c)
atomic theory (d) collision theory (e) gas laws

(20) Which of the following statements is not true about the reaction between dilute hydrochloric acid
and calcium trioxocarbonate(IV)? (a) It is faster after 3 seconds than it is faster after 10 seconds (b)
It slows down with time (c) It eventually stops (d) It proceeds at a constant rate (e) It causes a
loss of mass in reaction vessel contents.

(21) Which of the following are in accordance with Arrhenius equation?


(a) Rate of a reaction increases with increase in temperature
(b) Rate of a reaction increases with increase in activation energy.
(c) Rate constant decreases exponentially with increase in temperature
(d) Rate of a reaction decreases with decrease in activation energy
Use the figure below to answer questions 22 and 23:

(22) Activation energy of backward reaction is (a) E2 (b) E1 + E2 (c) E1 (d) E2-E1 (e) (E2)3

(23) Activation energy of forward reaction is (a) E1 x E2 (b) E2 (c) E1+E2 (d) E1 (e) E1/E2

(24) Mark the incorrect statements (a) Catalyst provides an alternative pathway to a reaction
mechanism (b) catalyst raises the activation energy (c) Catalyst lower the activation energy (d)
Catalyst alters enthalpy change of the reaction
(25) Write the rate equation for the reaction 2A + B C if the order of the reaction is
zero. (a) R = k[A] 0[B]0(b) R = k[A]1[B]2 (c) R = k[A]1[B]1 (d) R = k[A]3[B]
(26) In the presence of a catalyst, the heat evolved or absorbed during the reaction
--------- (a) increases (b) decreases (c) remains unchanged (d) may increase or
decrease

(27) The rate of reaction that does not involve gases is independent on
(a) temperature (b) pressure (c) catalyst (d) surface area (e) concentration

(28) The ratio of change in concentration of reactant or product to the time taken to
effect the change is (a) reaction rate (b) rate constant (c) concentration (d) gas
constant

(29) The unit of reaction rate is ---------------


(a) mols-1 (b) Ms-1k-1 (c) moldm-3s-1 (d) J/s (e) Jdm-3s-1

(30) The reactions that can be activated by light are called -------- reactions (a) silver
bromide
(b) Photosynthesis (c) exothermic (d) photochemical (e) endothermic
Theory
(1)

Explain at least four things that a medical chemist


should do in order to increase the rate of
producing C in the above chemical reaction.
(2)

Study the diagram above and use it to answer the


following:
(a) Find the value of activation energy
(b) What is the value of activation complex?
(c) Find the enthalpy (H) of the reaction

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